654 GEOLOGY OF OHIO. 
which makes the ware ring like a bell when struck, and is often a help 
in selling. 
The extent to which iron may be present without detriment is a 
point on which authorities do not agree. The Stourbridge clay of Eng- 
land, acknowledged to be the most refractory clay known, has 2.25 per 
cent. of iron on an average of 100 analysis, with extremes of 1.43 and 
3.63. Gros Almerode clay, has 2.12, Coblentz, 2.03, New Castle, 2.32, 
and yet all these clays are famous. Test mixtures of iron and pure 
kaolin have been run higher than this and have stood well, but as a 
general rule it is unsafe to rely for fine qualities on a clay with over 2 
per cent. of iron, particularly if the other impurities are developed in 
any amount. It isa well-known principle in chemistry that mixtures of 
bases are much more active fluxes than an equal amount of any one 
base; so with iron, its effects show worse when in presence of other flux- 
ing agents. 
The state in which the iron is present makes some difference; if as 
the sesquioxide, it takes more heat than when in the protoxide state to 
combine in the clay, for iron will only combine with silica in the pro- 
toxide state, and if that state is already developed, it is easier to com- 
bine the sand and iron than if in the other oxide. 
Sulphide of iron has a bad effect on the clay since its decomposition 
gives rise to the lower oxide of iron, besides the effect which the sulphur 
may have. 
Silicate of iron is also detrimental, since it melts ata comparatively 
low temperature. Ona piece of ware, iron in the uncombined state im- 
parts a buff or red color; when combination begins and progresses the 
ware is of a bluish-gray cast, deepening as the fusion of the iron pro- 
ceeds, and running to glassy black if much iron is present. 
Lime and magnesia act as fluxes on clays, but in Ohio fire clays 
the comparatively small amounts present makes them but little thought 
of as detrimental. They are probably present as silicates, and as these 
are readily fusible, their action is evidently unfavorable. When these 
bases are present as carbonates they combine at a higher temperature 
than iron or potash. The Milwaukee bricks, as already noted, are full 
of carbonates of lime and magnesia, and require a very hot burn, but 
when once the lime and silica combine they destroy the effect of 5 per 
cent. of iron, enough to make the clay perfectly black. A brick of this 
kind presents an even, fine-grained, vitrified appearance on its fracture. 
